In the semiconductor industry, a photolithographic method using visible light or ultraviolet light has been used as a technique for transferring, to a silicon substrate or the like, a fine pattern required for forming an integrated circuit consisting of the fine pattern. However, with accelerated miniaturization of semiconductor devices, the conventional photolithographic method has approached its limit. In the photolithographic method, a resolution limit of a pattern is about ½ of an exposure wavelength, and is said to be about ¼ of the exposure wavelength even by use of an immersion method. Even when an immersion method with an ArF laser (wavelength: 193 nm) is used, it is estimated that the limit of the resolution limit is about 45 nm. Then, as a next-generation exposure technique using a shorter resolution limit than 45 nm, EUV lithography is regarded as promising, which is an exposure technique using EUV light having a further shorter wavelength than the ArF laser. In the present specification, the EUV light indicates rays whose wavelength is in a soft X-ray region or a vacuum ultraviolet region, and specifically it indicates light rays whose wavelength is about 10 to 20 nm, particularly about 13.5 nm±0.3 nm (13.2 to 13.8 nm).
The EUV light is easily absorbed by any substance, and, in its wavelength, the refractive index of the substance is close to 1. Therefore, a refractive optical system such as a conventional lithography using visible light or ultraviolet light cannot be used. In the EUV lithography, therefore, a reflective optical system, that is, a reflective type photomask and a mirror are used.
A mask blank is a laminate before patterning, which is used for manufacturing a photomask. A mask blank for EUVL (EUV Lithography) has a structure in which a reflective layer that reflects EUV light and an absorption layer that absorbs the EUV light have been formed in this order on a substrate made of glass or the like.
A protective layer is typically formed between the reflective layer and the absorption layer. The protective layer is provided for the purpose of protecting the reflective layer so that the reflective layer can be prevented from being damaged by an etching process performed for the purpose of pattern forming on the absorption layer.
As the reflective layer, a multilayer reflective layer is typically used, in which low refractive layers having a low refractive index to the EUV light and high refractive layers having a high refractive index to the EUV light are disposed alternately to enhance the reflectivity when the surface thereof is irradiated with the EUV light. An example of such a multilayer reflective film may include a Mo/Si multilayer reflective film in which a plurality of bilayers each composed of a molybdenum (Mo) layer as the low refractive layer and a silicon (Si) layer as the high refractive layer have been disposed.
A material having a high absorption coefficient to the EUV light, such as a material containing chrome (Cr) or tantalum (Ta) as a main component, is used in the absorption layer.
In recent years, as optical specifications of a mask blank for EUVL, it has been increasingly required not only to have a high reflectivity but also to have a small in-plane distribution of a centroid wavelength (e.g. 13.5 nm) of reflected light in an EUV wavelength region. For example, it is highly likely that the in-plane distribution of the centroid wavelength will be required to satisfy 0.03 nm or less in the future. A high in-plane uniformity in the film thickness of a multilayer reflective film is required so that the in-plane distribution of the centroid wavelength can satisfy 0.03 nm or less.
Accordingly, the recent mask blank for EUVL has been required not only to have a high reflectivity but also to have the in-plane uniformity (small in-plane distribution) in the film thickness of the multilayer reflective film affecting the in-plane distribution characteristic of the centroid wavelength.
The in-plane distribution is an index of the largeness, and in the present specification, various in-plane distributions have the lower limit value of 0, unless otherwise specified.
Patent Literatures 1 and 2 propose, as one of factors that lower the in-plane uniformity in the film thickness of the multilayer reflective film, an in-plane film thickness distribution of a multilayer reflective film caused by an azimuth deviation in rotation of a substrate between at the start of deposition of low refractive layers and high refractive layers and at the end of the deposition thereof.
An object of Patent Literature 1 is to suppress the in-plane film thickness distribution of a multilayer reflective film caused by an azimuth deviation in rotation of a substrate between at the start of deposition of low refractive layers and high refractive layers and at the end of the deposition thereof. In order to attain the object, according to Patent Literature 1, a change with time in a deposition rate is estimated for each of low refractive layers and high refractive layers to set deposition conditions for the low refractive layers and the high refractive layers.
Patent Literature 1 discloses two modes for setting the deposition conditions of the low refractive layers and the high refractive layers based on the change with time in the deposition rate estimated for each of the low refractive layers and the high refractive layers. In one of the modes, the rotation number of a substrate during the deposition of each of the low refractive layers and the high refractive layers constituting the multilayer reflective film is set to 1 rpm or more and 80 rpm or less, and the rotation number of the substrate is controlled for each layer (each of the low refractive layers and the high refractive layers) constituting the multilayer reflective film so as to reduce the rotation number of the substrate gradually. In the other mode, the rotation number of the substrate during the deposition of each of the low refractive layers and the high refractive layers constituting the multilayer reflective film is set to 80 rpm or more and 300 rpm or less, and the rotation number of the substrate is fixed or substantially fixed.
The aforementioned “rotation number of the substrate” can be regarded to mean “rotation speed of the substrate” because it is designated by rpm as its unit.
On the other hand, an object of Patent Literature 2 is to suppress an in-plane film thickness distribution of a multilayer reflective film caused by an azimuth deviation in rotation of a substrate between at the start of deposition of low refractive layers and high refractive layers and at the end of the deposition thereof. In order to attain the object, according to Patent Literature 2, the azimuth (deposition start position) at the start of deposition of each layer (each of the low refractive layers and high refractive layers) is changed to form the layer. Thus, the deposition start position of each layer is dispersed. In this manner, the in-plane distribution of a diffusion layer formed between the layers is suppressed from being accumulated, so that the in-plane film thickness distribution of the multilayer reflective film can be suppressed.
In the aforementioned one mode for setting the deposition conditions of the low refractive layers and the high refractive layers in Patent Literature 1, it is suggested that the rotation number of the substrate (the rotation speed of the substrate) is controlled for each layer (each of the low refractive layers and the high refractive layers) constituting the multilayer reflective film. However, it is practically difficult to control, as shown in FIG. 6 of Patent Literature 1, the rotation number of the substrate (the rotation speed of the substrate) for each layer of 80 or more layers constituting the multilayer reflective film. In addition, even if it is possible to perform such control, it will spend a long time for the control until the rotation number of the substrate (the rotation speed of the substrate) for each layer is stabilized. Thus, the throughput is lowered. Further, when the rotation number of the substrate (the rotation speed of the substrate) is changed for each layer, a risk of occurrence of defects may increase due to a dynamic action caused by the change of the rotation speed.
On the other hand, the other mode for setting the deposition conditions of the low refractive layers and the high refractive layers in Patent Literature 1, it is suggested that the rotation number of the substrate (the rotation speed of the substrate) during the deposition of each layer constituting the multilayer reflective film is controlled to be 80 rpm or more and 300 rpm or less. However, it is practically difficult to control the rotation of the substrate at such a high speed. In addition, even if such control can be carried out, a risk of occurrence of defects may increase.
Also in Patent Literature 2, it is practically difficult to perform control to change the deposition start position of each layer as to the 80 or more layers constituting the multilayer reflective film.
The aforementioned problem relating to the in-plane distribution of film thickness during formation of a multilayer film can be also a problem in a multilayer film-deposited substrate for other applications severely required as to the in-plane uniformity of the film thickness.